Diagnostic devices have been provided in the past for conducting immunoreactions in one or more test wells to determine the qualitative existence of pregnancy or an infectious disease. Such devices commonly include a filter membrane at the bottom of the well to separate out free, unreacted immunoreagents, from complexed immunoreagents. (The former pass through the membrane.) Underneath such membranes is preferably located on absorbent material to absorb the liquid of the reaction mixture. It has been conventional in such devices to provide for control of liquid flow, so as to first retain the liquid in the well above the membrane for an incubation period, and then to allow flow through the membrane. Previously provided mechanism for controlling the flow include an aperture in the housing for the absorbent material underneath, and seals for opening and closing the aperture. When the aperture is open, normal liquid wetting causes the liquid to flow though the membrane into the absorbent material. Such seals, although effective, require careful tolerances and assembly techniques, so as to insure the apertures are normally closed, while at the same time are openable using manual forces easily available to the user.
Thus, prior to this invention there has been a need to provide liquid flow control for such devices that do not have such high tolerance requirements, while at the same time allowing easy use by the operator.
A recent development towards this end is that described in U.S. Pat. No. 4,734,262 by Bagshawe. In this device, a non-absorbent seal is provided at the bottom of the well, between the opening in the well and the filter membrane that is combined with an absorbent material. The device is constructed to provide relative movement in the form of reciprocation, between the seal and the opening, but not between the seal and the filter membrane. That is, both the seal and membrane are caused to deflect away from the opening, such as by applying pressure to the top chamber, allowing liquid to pour out onto the filter membrane. Such a construction has several disadvantages. One is that the seal covers the surface of the filter membrane at the tip, thus decreasing the area available for filtration, and most importantly, it prevents the filter from being observed through the opening for a detectable change indicative of a positive reaction. A second disadvantage is a requirement that there be a positive driving pressure to open the seal. This requires an auxiliary instrument, such as a vacuum source, besides what can be done using just the operator's own manipulation.